Accurate analysis of deepwater pay zones determines commercial reserves.

The estimation of hydrocarbon production potential in thinly laminated reservoirs is critical in today's deepwater plays. In these environments, e.g., Gulf of Mexico, west of Africa and offshore Brazil, thinly laminated turbidite sands are associated with large hydrocarbon accumulations. This is especially true in offshore Angola where one of the most significant critical challenges in deepwater exploration and appraisal programs is the accurate estimation of reserves in potential reservoir formations. These laminated sand packages are difficult to identify and to evaluate accurately with conventional, coaxial induction-tool data.

In the complex and high-cost operating environment of offshore West Africa, formation evaluation methods and accompanying interpretation and analysis techniques have evolved rapidly to meet the economic and risk demands of reservoir evaluation. The reservoirs can be very complex and heterogeneous, ranging from massive thick sands to highly laminated sand/shale sequences and are typically unconsolidated. If the thickness of the sand/shale laminae is below the vertical resolution of the standard wireline data, then a petrophysical evaluation of the hydrocarbon-bearing sand laminae using conventional wireline data is problematic.

Interpretation methods

Normally associated with conductivity much larger than that of the hydrocarbon bearing sands, laminar shales dominate the horizontal resistivity of shaly-sand reservoirs, leading to low resistivity readings. Conventional interpretation, which normally deals with a single resistivity measurement, prevents the analyst from producing a true reservoir characterization, i.e., the characterization of the actual reservoir formed by the sand lamina.

A multi-component induction logging instrument can provide additional data to accurately estimate the reservoir resistivity anisotropy. This additional log data allows for an improved petrophysical evaluation of thinly bedded, laminated reservoirs.

Logging tool

One such technology enabling determination of reservoir properties in low contrast formations as described above is the 3D Explorer (3DEX) induction logging instrument. The new multi-component induction logging instrument provides three orthogonal measurements sensitive to resistivity anisotropy. Measurements are made in the transverse (parallel to tool axis) directions, and Z measurements in the traditional (perpendicular to tool axis) direction.

The instrument provides direct measurements of electrical anisotropy of formations penetrated by a wellbore. In hydrocarbon-bearing, laminated sand/shale sequences, formation resistivity measured parallel to the bedding planes is usually significantly lower than resistivity normal to the bedding, due to the relatively high conductivity of the shales and higher resistivity of the sands.

Measurements made parallel to the bedding planes record horizontal resistivity (Rh), where the laminae appear to the measurement instrument as parallel resistor elements. The bulk measurement of horizontal resistivity is therefore dominated by the high conductivity shales and is insensitive to the presence of hydrocarbons in the sand laminae. The multi-component induction instrument records additional responses that are sensitive to vertical resistivity (Rv), where the laminae are seen as resistor elements in series. This parameter is dominated by the high resistivity beds and is, in consequence, much more sensitive to the presence of hydrocarbon-bearing sand laminae, providing a more robust detection and quantification of hydrocarbons.

The tool's measurement allows for accurately determination of water saturation and volume of laminar shale in thinly bedded sand-shale sequences. It can differentiate potentially productive from unproductive zones and greatly reduce uncertainties in formation evaluation. The service enables the operator to find pay hidden in low resistivity/low contrast environments. These environments are often overlooked and may typify some marginal fields such as those found in Nigeria. This new logging technology helps operators today to book additional pay.

Higher reserves have been computed utilizing this data producing results consistent with field production history.